In the transmission of digital signals, bit stuffing has become a common way of synchronizing lower bit rate signals for transmission at a higher bit rate. One bit stuffing arrangement employs two stuffing bit positions in each channel frame and is commonly referred to as positive and negative stuffing. One such channel frame employing positive and negative stuffing is shown in FIG. 1 of the Drawing and includes the two stuffing bit positions S1 and S2. Generation of such a channel frame is disclosed in copending U.S. patent application Ser. No. 769,427, filed Aug. 26, 1985. When the incoming data bit rate is at its nominal rate, one of the stuffing bit positions includes a data bit and the other a stuffing bit. If the incoming data bit rate is greater than the nominal rate, then occasionally, as required, both of the stuffing bit positions S1 and S2 include data bits. This is commonly referred to as negative stuffing. On the other hand, if the incoming data bit rate is less than the nominal rate, then occasionally, as required, both of stuffing bit positions S1 and S2 include stuffing bits. This is commonly referred to as positive stuffing.
A serious problem with such a positive and negative stuffing arrangement is that the so-called waiting time jitter caused by the bit stuffing is too large. Waiting time jitter resulting from bit stuffing is described in Transmission Systems for Communications, 5th Edition, 1982, Bell Telephone Laboratories, Incorporated, pages 692-699. Also see an article by D. L. Duttweiler entitled "Waiting Time Jitter", Bell System Technical Journal, Vol. 51, No. 1, January 1972, pages 165-207, and an article entitled "Jitter Characteristics of Pulse Stuffing Synchronization", IEEE Proceedings International Conference on Communications, June 1968, pages 259-264. In the positive and negative stuffing arrangement, a nominal stuffing ratio of one (1) results since one of the stuffing bit positions nominally includes a stuffing bit. Thus, as explained in the above-cited articles, the so-called waiting time jitter value is very large, which is extremely undesirable.
It is known that a positive stuffing scheme which causes the stuffing ratio to be reasonably greater than zero (0) and less than one (1) will significantly reduce waiting time jitter.
It is also known that a synchronous stuffing arangement does not generate waiting time jitter.